Summary Sensory perception depends not only on the physical properties of the stimulus, but is also modulated by internal states including attention, hormonal status and metabolism as well as experience and environmental stimuli. How does the nervous system integrate this comprehensive information to select an appropriate behavioral output for a particular sensory input? To address this question, it is crucial to link specific neuronal activity to behaviors or percepts. The olfactory system provides a unique opportunity to study how genetically defined circuits give rise to specific behaviors. Moreover, the perceived valence of odors and the corresponding behavioral reactions can be state- and context-dependent, making it an ideal model to study sensory modulation. Here, I propose to use a powerful new model involving a naturally occurring switch in odor valence, to reveal the neuronal coding strategies underlying sensory modulation. In this model, the valence response of female mice to a male volatile social cue significantly depends on the estrus-state. Moreover, the observed change in behavior requires a defined olfactory receptor, which is expressed in the main olfactory epithelium. This naturally occurring switch in odor valence provides a rare opportunity to study a non-conditioned, potentially inducible change in perception. Specific Aim 1 will determine the role of hormones in the observed odor valence modulation. Specific Aim 2 will use a newly developed head-fixed odor valence assay that combines physiological recordings in awake mice with simultaneous behavioral analysis to dissect the peripheral and central mechanisms of state dependent valence modulation. This work will be a key first step to identify the neural circuitry underlying modulation of olfactory perception and will shed light on how specific perceptual features are encoded in the nervous system.